The present invention relates generally to electronic ballasts used for powering gas discharge lamps. More particularly, the present invention pertains to methods and circuits for providing overvoltage protection and automatic lamp re-striking in an electronic ballast.
Electronic ballasts for gas discharge lamps, e.g. fluorescent lights, are well known in the prior art. Electronic ballasts can provide, among others, the means to ignite and operate the gas discharge lamps.
Gas discharge lamps are lit through a variety of methods. For exemplary purposes, one method requires the lamp, having an elongated tube with a phosphor coating on the inside surface, to be subjected to a large voltage differential between its terminals. This large voltage differential is sufficient to generate an electrical pathway between the terminals (the voltage differential is greater than the breakdown voltage between the lamp terminals). The resultant current flowing between the terminals excites gaseous atoms, already present in the tube of the lamp, which in turn causes the gaseous atoms to release photons. These photons are outside of the visible spectrum, typically, in the ultraviolet range. These ultraviolet photons bombard the atoms comprising the phosphor coating of the tube and cause the phosphor coating to release photons which are in the visible spectrum. In this way visible light is produced.
The ballast occupies an integral role in this process. The ballast supplies the means to ignite the lamp through the process detailed above. Once the lamp is ignited, the ballast also regulates the electrical current that flows through the lamp. Without the regulation efforts of the ballast, the current demanded by the lamp would be significant because once the lamp begins to operate it presents very little electrical resistance. If there was not a mechanism to curtail the current demanded by the lamp, the lamp would be impractical to use.
Of particular import is the ability of the ballast to reliably ignite, or re-ignite, the lamp after the lamp malfunctions or is replaced. Ideally, the ballast should successfully ignite the lamp with only one attempt but it is not unusual for a ballast to make a series of ignition attempts before the lamp actually ignites. This succession of ignition pulses engenders the ballast ignition system with a degree of robustness.
However, it is also desirable for the ballast to recognize when a lamp is faulty and cannot be lit or when no lamp is present. In either case it would be advantageous for the ballast to appreciate that further ignition attempts will be fruitless. Unfettered re-ignition attempts can pose safety risks to those exposed to the lamp fixture because the ballast must generate a significant voltage to induce the lamp to ignite. Moreover, continuous ignition or re-ignition attempts needlessly stress the ballast and can lead to premature component fatigue and eventual failure. Consequently, a ballast that can generate a series of ignition pulses to effectively ignite a lamp and can also diagnose when further ignition attempts are ill advised is desirable.
No less crucial than ignition concerns is a the ability of the ballast to guard against potentially damaging overvoltage conditions, such as when the lamp experiences input arcing or unsuccessful ignition attempts. To effectively forestall damage from overvoltage conditions, the ballast must expeditiously recognize and suppress the overvoltage condition before irreparable damage occurs. As with unnecessary ignition attempts, overvoltage conditions are deleterious to the ballast because the ballast's components are stressed. Prolonged and/or excessive overvoltage conditions can stress the components until they fail.
As discussed above, when a ballast attempts to ignite a lamp, a large voltage differential is presented across the lamp terminals. Typically, this voltage differential is applied across the terminals by an inverter. For a myriad of reasons a lamp may not ignite even with a sufficient voltage differential across its terminals—alternatively, ignition may not even be possible if no lamp is present. If the differential were allowed to build beyond this point the ballast may be damaged, in addition to posing dangers for individuals working around these ballasts. To prevent this from happening the ballast needs an overvoltage protection mechanism to disable the inverter or otherwise safely dissipate the accumulated voltage differential. Additionally, to effectively protect the ballast, the overvoltage protection mechanism must rapidly address this overvoltage condition.
Thus, a contentious relationship exists between providing a voltage differential large enough to effectively ignite the lamp, overstressing the ballast by exposing the ballast to extreme voltages or high voltages for prolonged periods of time, and mitigating potential hazards to persons dealing with the ballast. As such, a ballast capable of expertly managing these concerns, particularly any associated overvoltage conditions that may arise, is paramount to safe and reliable ballast operation.
The prior art has not left these concerns unaddressed. Conventional ballasts disclosed in the prior art handle overvoltage conditions by completely disabling the inverter or retarding the output of the inverter. Prior art ballasts also teach systems having multiple re-strike ignition capabilities that can be limited to a predetermined number of attempts. For example U.S. Pat. No. 7,015,652 issued to Shi discloses one such ballast. Shi teaches a ballast having an overvoltage protection system with multiple re-strike capabilities that can be controlled. However, the prior art does not include a ballast that has a reliable, safe, automatic re-striking capability following an overvoltage shutdown condition, an overvoltage protection mechanism that responds with sufficient speed to protect the ballast regardless of the cause of the overvoltage condition, and the ability to recognize when further re-ignition attempts should cease, e.g. a faulty lamp.
What is needed, then, is a ballast that provides overvoltage protection and re-ignition systems that cooperate to produce an effective, reliable ballast in a simple implementation so that measured automatic re-ignition attempts are made after the ballast has reacted to an overvoltage condition.